This invention relates generally to oscillators and more particularly to crystal controlled overtone oscillator circuits that prevent oscillations at undesired crystal overtone frequencies.
There are many prior art applications that prevent oscillations of oscillators at undesired frequencies. In particular there are many prior art crystal oscillators that prevent an overtone crystal oscillator from oscillating at undesired overtone frequencies. Prior art techniques for reducing spurious oscillation typically require the usage of reactive components, especially inductive elements, to inhibit oscillation at an undesired frequency.
For example, U.S. Pat. No. 3,845,410 to Steel for a "Crystal Oscillator Having Spurious Oscillation Suppression Circuit" teaches a Colpitts type oscillator having an inductive and resistive element chosen to provide power dissipation at undesired spurious frequencies and to allow only minimal losses to occur at the desired operating frequency. Similarly, U.S. Pat. No. 3,875,533 to Irwin et al discloses a Colpitts oscillator circuit having an external transformer to minimize undesired oscillations.
In the effort to reduce the physical size of circuitry, an oscillator circuit that is capable of oscillating at a crystal overtone while not requiring an external inductor to suppress undesired oscillations would be an improvement.